The present invention relates generally to an apparatus for effecting wide range control of a delivery means responsive to at least two independent controls and more particularly to an apparatus for effecting wide range control of a pump.
In many instances, it is necessary to deliver a substance to a desired point at a delivery rate that is responsive to at least two independent parameters. For example, a chemical feed pump may have both a stroke adjustment and a speed adjustment. In certain of these instances, the required delivery rate is proportional to the value of a single signal having a wide operating range, for example, from 100% pump output down to 1% of maximum pump output. Typically, neither the stroke adjustment nor the speed adjustment are capable of accurately controlling the pump over such a wide range. For example, these adjustments may be reasonably accurate over only a control range of 10 to 1. Operating under these constraints, it has been necessary to devise techniques to permit wide range control of such delivery apparatuses suitable for response to a single control signal.
To date, one way this problem has been overcome is by applying the control signal to the first control to effect a first range of reduction in capacity, while holding the second control at its maximum position. After the first control reaches a predetermined level, the control signal is then switched to the second control, multiplied by a compensating factor, and then utilized to reduce the second control to a predetermined level. Thus, in the above example the pump stroke could be reduced from a 100% down to 10% while maintaining pump speed at 100%. The pump stroke could then be maintained at 10% while a multiplying signal of 10 was applied to the control signal permitting reduction of pump speed from 100% down to 10%.
Such a control scheme is also illustrated in U.S. Pat. No. 3,493,167 to Akerhielm, et al. This patent shows a control system for regulating the capacity of centrifugal compressor by varying the compressor speed to effect a first range of reduction in capacity and thereafter maintaining compressor speed constant while varying a compressor guide vane adjustment to effect an additional capacity reduction.
Although such prior systems have proved effective in obtaining wide range delivery from a single control signal, their degree of accuracy has not always been satisfactory. Applicant has discovered that by devising a scheme whereby both controls may be modified simultaneously, accuracy may be significantly improved.